Devices for overcoming draft problems frequently associated with chimneys as well as vent systems having a motor-driven fan or other gas-propelling device are well known in the art and are classified as draft inducers. Also known in the art are powered vent fans having a motor-driven, centrifugal fan wheel in a housing that can be mounted in association with a vent pipe to create a mechanical draft, as may be required by new high-efficiency furnaces, unit heaters, and other fuel-burning appliances. The powered vent fan assures a constant, uninterrupted flow of draft and flue gases through the fuel-burning appliance and its vent system. Both draft inducers and powered vent fans have in common a housing and a motor-driven fan or blower wheel. Additionally, both draft inducers and powered vent fans operate by creating regions of respectively higher and lower pressures to cause fluid to flow towards or away from a specific appliance or unit.
An early example of a fan connectable into a flue pipe is shown in U.S. Pat. No. 886,268 wherein the fan is manually driven.
Other known prior art draft devices which may be mounted inline in a vent pipe of a vent system are shown in U.S. Pat. Nos. 2,588,012 and 2,617,371.
Inline vent fans typically allow for the inlet and outlet of the fan to be positioned along a common axis and therefore can be placed within an existing pipe, such as seen in U.S. Pat. No. 4,750,433. However, in some instances inserting vent fans into existing pipes can be costly, time consuming and connections therebetween may be prone to leaking. Additionally, the vent fan may not operate efficiently if the inlet and outlet are not sealed to prevent recirculation of the fluid because the distinct zones of respective positive and negative pressure become intermixed. Furthermore, such inline vent fans tend to lose efficiency due to drag and friction from within the fan itself as the fluid contact different parts and regions of the fan.